Method of preparing magnetically responsive carrier particles

ABSTRACT

FERROMAGNETIC CARRIER PARTICLES HAVING UNIFORM SURFACE AND TRIBOELETRIC PROPERTIES CAN BE PREPARED BY TREATMENT OF COMMERCIAL IRON POWDERS IN AN AQUEOUS ACID SOLUTION, FOLLOWED BY REMOVAL OF ACID, RINSING AND CONTROLLED DRYING TO INCLUDE OR EXCLUDE OXIDATION. THE RESULTANT PARTICLES CAN SUBSEQUENTLY BE OVERCOATED WITH A THIN, UNIFORM, CONTINUOUS FILM OR A NONFERROUS MATERIAL SUCH AS A METAL OR A RESINOUS MATERIAL. THESE CARRIER PARTICLES ARE USEFUL FOR APPLYING ELECTROSCOPIC TONER MATERIAL TO ELECTROSTATIC LATENT IMAGES.

United States Patent O 3,632,512 METHOD OF PREPARING MAGNETICALLYRESPONSIVE CARRIER PARTICLES Howard A. Miller, Rochester, N.Y., assignort Eastman Kodak Company, Rochester, N.Y. N0 Drawing. Filed Feb. 17,1969, Ser. No. 799,966 Int. Cl. G03g 9/02 US. Cl. 25262.1 2 ClaimsABSTRACT OF THE DISCLOSURE This invention relates to electrophotography,and more particularly, to magnetically attractable carrier particlesuseful in the magnetic-brush type development of electrostatic latentimages.

Electrophotographic imaging processes and techniques have beenextensively described in both the patent and other literature, forexample, US. Pats. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809;2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 andmany others. Generally, these processes have in common the steps ofemploying a normally insulating photoconductive element which isprepared to respond to imagewise exposure with electromagnetic radiationby forming a latent electrostatic charge image. The electrostatic latentimage is then rendered visible by a development step in which thecharged surface of the photoconductive element is brought into contactwith a suitable developer mix.

One method for applying the developer mix is by the well-known magneticbrush process. Such a process can utilize apparatus of the typedescribed, for example, in U.S. Pat. No. 3,003,462 and often comprises anonmagnetic rotatably mounted cylinder having fixed magnetic meansmounted inside. The cylinder is arranged to rotate so that part of thesurface is immersed in or otherwise contacted with a supply of developermix. The granular mass comprising the developer mix is magneticallyattracted to the surface of the cylinder. -As the developer mix comeswithin the influence of the field generated by the magnetic means withinthe cylinder, the particles of the developer mix arrange themselves inbristle-like formations resembling a brush. The bristle formations ofdeveloper mix tend to conform to the lines of magnetic flux, standingerect in the vicinity of the poles and lying substantially flat whensaid mix is outside the environment of the magnetic poles. Within onerevolution the continuously rotating cylinder picks up developer mixfrom a supply source and then returns part or all of this material tothe supply. This mode of operation assures that fresh mix is alwaysavailable to the surface of the photoconductive element at its point ofcontact with the brush. In a typical rotational cycle, the rollerperforms the successive steps of developer mix pickup, brush formation,brush contact with the photoconductive element, brush collapse andfinally mix release.

In magnetic-brush development of electrostatic images, the developer iscommonly a triboelectric mixture of fine toner powder comprised of dyedor pigmented thermoice plastic resin with coarser carrier particles of asoft magnetic material such as ground chemical iron (iron filings),reduced iron oxide particles or the like. The conductivity of theferromagnetic carrier particles which form the bristles of a magneticbrush provides the effect of a development electrode having a very closespacing to the surface of the electrophotographic element beingdeveloped. By virtue of this development electrode effect it is to someextent possible to develop part of the tones in pictures and solidblacks as well as line copy. This ability to obtain solid areadevelopment with magnetic brush developing sometimes makes this mode ofdeveloping advantageous where it is desired to copy materials other thansimple line copy. One difficulty with such counter-electrode developmentis that the exposure latitude obtainable is limited. Consequently, forcertain applications, it is desirable to suppress the counter-electrodeeffect in order to obtain improved exposure latitude. One method ofsuppressing the counter-electrode effect is to use a carrier materialwhich has a high electrical resistance.

Presently available commercial iron powders, however, are not at alluniform in regard to the properties of conductivity and resistivity. Ingeneral, the materials now available have too high resistivity to givegood solid area development and are too conductive to entirely suppressthe counter-electrode effect. One reason for the lack of uniformity inthe electrical properties is that the particles carry surface dirt, suchas grease, oil, and other contaminants. It has been suggested in theprior art that typical iron carrier particles can be treated withmethanol, isopropanol, and other alcohols to rid the iron of grease,oil, etc. Iron so treated is generally referred to as alcoholized iron.

However, the prior art has not recognized the heterogeneous nature ofthe intimate surface of the iron powders used in this art. In addition,no recognition has been given to the resulting complications inmagnetic-brush processing which arise from using some of the presentlyavailable materials. The extraneous surface dirt found on the availableiron particles is only part of the problem. In addition to the dirt, theiron particles invariably carry a non-uniform distribution of iron oxideand oftentimes pyrites such as iron sulfide, as well as other ironcompounds. When materials of this type are used in conventional magneticbrushes, the iron carrier is in continuous motion and the constantfriction of the particles against one another and against the variousmechanical parts gradually remove bits of this nonuniform surfacematerial. The attrition will, of course, vary from point to point on aniron particle depending on the nature of the deposit, its thickness,friability, adhesion to the underlying structure, and the like. Thus,with the available carrier materials, a progressive change occurs in thecharacter of the iron surface during use. Correspondingly, the nature ofthe admixed toner also changes as it becomes contaminated with the fineparticles which have separated from the surface of the carrier.

Accordingly, there is a need for iron carriers which will present a morenearly homogeneous surface and which will have stable triboelectric andother properties even during continued use. Furthermore, there is a needfor iron carrier patricles which will enhance the counter-electrodeeffect and thus improve the solid-area type development obtainable inmagnetic brush processing. Also, there is a need for iron carrierparticles which can repress the counter-electrode effect so thatfringing development will be induced with its accompanying improvementin exposure latitude.

It is, therefore, an object of this invention to provide a new simpleeconomic process for preparing magnetically responsive iron carrierparticles having a homogeneous surface.

It is another object of this invention to provide a new process forpreparing magnetically responsive carrier particles which process isadapted for forming particles of high surface conductivity or lowsurface conductivity, as desired.

A further object of this invention is to provide new developingcompositions for use in magnetic brush development.

These and other objects and advantages are accomplished in accordancewith this invention by the treatment of iron powder with an aqueoussolution of acid followed by removal of residual acid and waste productsevolved with subsequent controlled drying of the iron powder underpreselected conditions of temperature, relative humidity and ambientatmosphere. In addition, the dried powder can be further treated byapplying one or more thin continuous layers of an additional nonferrousmaterial.

The carrier materials which are suitable for treatment in accordancewith this invention include ferromagnetic materials such as iron powderin varoius forms. The phrase iron powder as used herein is meant toinclude a wide variety of particulate, magnetically responsive materialthe surface of which can be readily oxidized and includes material insuch forms as reduced iron oxide bits; particles produced by atomizationof molten metal and subsequent cooling of the droplets; particlesproduced by grinding, milling, filing, turning, etc.; as well asparticles of iron alloys having oxidizable iron on the surface thereofsuch as stainless steel and iron alloys containing nickel and/or cobalt.The ferromagnetic carrier particles used can vary in size and in shapewith useful results being obtained with average particle sizes of fromabout 1200 to about 40 microns. Particularly useful results are obtainedwith average particle sizes from about 600 to about 60 microns. The sizeof the carrier particles used will, of course, depend upon severalfactors, such as the type of images ultimately developed, the desiredthickness of any subsequent coatings, etc.

Commercially available iron powder is first subjected to an acidtreatment in accordance to this invention. This acid treatment can becarried out using a dilute aqueous solution of the acid. A wide varietyof acids can be used such as sulfuric, hydrochloric, nitric, and othermineral acids. Sulfuric acid is a preferred material, by virtue of thefact that it is inexpensive, readily available, and only slightlyvolatile. In addition, certain organic acids, such as acetic acids,e.g., trichloroacetic acid, etc., are also useful in certainapplications. In general, the acid concentrations which are useful canvary from about l-normal to about 3-normal. Higher or lower acidconcentrations can sometimes be useful because of the quantity of ironbeing treated, the average particle size or other such variables. Thestep of acid treatment of the iron particles is in no way restricted toany particular method of applying the acid. For example, spraying,percolation, and/or other means can conveniently be used to accomplishthe acid treatment step. One very convenient procedure is to form aslurry of the iron powder in dilute acid for a period of time which willvary with the nature and concentration of the acid used, the agitationapplied, the required degree of attack by the acid, the particle size ofthe iron powder, as well as the character and thickness of the surfacematerial which is to be removed. When using commercial iron powders ofaverage size, from about 100 microns to about 400 microns, it isgenerally sufficient to agitate the iron powder in a 5% aqueous solutionof, for example, sulfuric acid for about one minute at about 25 C.

If necessary to stop the reaction of the acid on the carrier powder, analkaline solution can be added to the acid slurry. Alternatively, thereaction can be checked by dilution with cold water. However, it isgenerally simpler and more efficient to start with an acid concentrationof the appropriate value such that the reaction rate will besuificiently slowed down at the end of the treatment period, so as tofacilitate settling of the iron powder and removal of the supernatantliquid. As mentioned above, the acid concentration required will varywith different iron powders, depending on the amount used and theaverage particle size, the nature of the surface on the startingmaterial, etc.

Despite the harmful effects of the various surface contaminants found oncommercial iron powders, such contaminants constitute only a very smallfraction of the total weight of the powder. In most cases, the reductionin weight resulting from removal of undesired surface contaminants andincidental solution of elementary iron in the treatment of commercialiron powders by the method of this invention, is of the order of about 1to about 2% by weight or less. Treatment need not be extended beyond thedegree necessary to remove the heterogeneous surface contaminants. Nofurther reduction in surface contaminants nor improvement in surfaceuniformity is observed by continuing treatment until the loss ofmetallic iron is about 2% by weight. Preferably, the metallic iron lostby dissolution should be less than about 1% by weight of the ironpowder. In fact, unduly prolonged treatment not only results in needlessconsumption of acid and dissolution of iron but, in some instances, canproduce deleterious results, by exposing occluded particles ofacid-insoluble material.

After processing the iron powder in the aqueous acid solution, the acidis removed and the iron is washed by percolation or other means toinsure removal of any residual acid. A large part of the materialremoved from the surface of the powder is often insoluble andconsequently, decantation washing is a preferred method of eliminatingboth the residual acid and the suspended products of the reaction. Asimple decantation procedure can be carried out by slurrying the iron inclean cold water followed by a settling period to permit the ironparticles of suitable carrier size to settle to the bottom of theliquid. After setling the supernatant liquid is rapidly decanted suchthat the unwanted contaminants are carried off with the liquid. Thiswashing operation can be repeated until the rinse water is clear oruntil an established level of residual turbidity is reached.

The process of this invention can be used to prepare iron particleshaving very high surface conductivity. The highly conductive particles,made in accordance with this invention, show surface resistance valuesseveral orders of magnitude lower than the best of the commerciallyavailable iron powders when measured in magnetic brush conformationunder standardized testing conditions. For purposes of comparison, theresistance of the carrier particles is measured in a standard resistancetest. This test is conducted each time using a 15 g. quantity of carriermaterial. A cylindrically shaped magnet having a circular end of about6.25 square centimeters in area is used to attract the carrier and holdit in the form of a brush. After formation of the brush, the bar magnetis then positioned with the brush-carrying end approximately parallel toand about 0.5 cm. from a burnished copper plate. The resistance of theparticles in the magnetic brush is then measured between the magnet andthe copper plate. In general, the uncoated conductive particles made inaccordance with this invention have a resistance of less than about 50ohms, with especially preferred materials having a resistance of lessthan about 20 ohms.

Metallic iron is a very good electrical conductor and the cleanelemental iron surface of the carrier iron prepared by this process ofthis invention results in a highly conductive iron powder as mentionedabove. However, iron is also a highly reactive metal and readilyoxidizes in air. Therefore, it is of utmost importance when makinghighly conductive carrier material by this invention to repressoxidation during the washing and drying steps as well as duringsubsequent storage. The repression of oxidation can be accomplished inany of several ways. One useful Way to repress oxidation is by theaddition of an antioxidant or reducing agent to the wash water andparticularly to the last several quantities of wash water. A variety ofknown antioxidants or reducing agents can be used to inhibit oxidationin the subsequent drying operation. For example, one-half percent ofhydrazine sulfate or sodium nitrite is usually quite effective for thispurpose. Oxidation can also be minimized by the use of one or asuccession of rinses in a Water-miscible organic solvent such as acetoneor a lower alcohol like methanol, ethanol and isopropanol. Such solventsremove the residual water from the iron and speed subsequent drying.Additionally, the temperature and relative humidity of the drying airshould also be carefully controlled. One reason for the careful controlis that warming of the iron powder will encourage oxidation whilesimilarly, if the iron is cooled too rapidly such that the temperaturedrops below the dew point, condensation of water on the iron will alsoencourage surface oxidation. Thus, when preparing conductive carriers,it is necessary that drying be carried out under temperature andrelative humidity conditions which discourage the formation of surfaceoxide.

In general, the iron powder processed according to this highlyconductive embodiment of the invention can be dried by agitatingcontinuously in a current of Warm air. Generally, the temperature of theair is in the range of from about 15 to about 40 C. with a preferredtemperature of from about 20 to about 30 C. In addition, the dryingtemperature should be kept above the dew point of the ambient atmosphereto avoid unnecessary condensation. Furthermore, the relative humidity ofthe drying air should be kept low. Generally, good results can beobtained with relative humidity values of less than about 30% andpreferably the relative humidity is less than about 20% Under theseconditions of temperature and humidity, the resulting powder shows veryhigh surface conductivity. If, however, even higher surfaceconductivities are required, the iron can be dried in an inert,substantially oxygen-free atmosphere such as hydrogen, nitrogen, etc.After drying, the iron must be protected from oxidation until ready foruse. This can be readily accomplished by storing it in an inertatmosphere in a sealed container. When used, the carrier particles aremixed with a toner material of suitable triboelectric properties whichtoner provides a physical coating that effectively lessens any furtheroxidation on the carrier during use. As a result, it is possible tomaintain the improved surface conductivity of the carriers of theinvention during extended periods of use in magnetic brush apparatus.

Of course, it is also possible to stabilize the improved surfaceconductivity of particles prepared in accordance with this invention byadditional surface treatment of the carrier. One particularly usefulsurface treatment involves overcoating each carrier particle with a thincontinuous layer of a nonferrous electrically conducting metal which ismore resistant to aerial oxidation than iron. The materials useful forcoating or plating onto the present conductive carriers are typicallynonferrous metals which are substantially more resistant to surfaceoxidation than iron and iron-alloy. Suitable coating materials having aresistance to aerial oxidation greater than that of iron include thosemetals in Groups VIa, VIII, Ib and 11b of the Periodic Table (Cotton andWilkinson, Advanced In organic Chemistry, 1962, page 30). Particularlyuseful metals are cadmium, chromium, copper, gold, nickel, silver, zinc,and the platinum group elements which include ruthenium rhodium,palladium, osmium, iridium and platinum as well as mixtures of alloy ofany of these. Most of these metals are more electronegative than theiron starting material which property is advantageous in certain coatingprocedures. With other coating procedures, the metals moreelectropositive than iron can be useful such as chromium, zinc andcadmium.

The useful conducting metals all have a greater corrosion resistance orresistance to aerial oxidation than does iron. The terms corrosionresistance or resistance to aerial oxidation all have reference to theability of a metal to withstand oxidative-type corrosion which impairselectrical conductivity. In general, the type of corrosion which shouldbe avoided is continuous aerial oxidation or rapid aerial oxidationwhich substantially reduces the electrical conductivity of a metal. Inparticular, these terms have reference to corrosion induced by exposureto air, carbon dioxide, water, ozone, etc., and do not have particularreference to the chemical attack of solutions of strong acids or bases,etc. Coatings of this type are further described in H. A. Miller US.application Ser. No. 799,967, filed Feb. 17, 1969, co-filed herewith,and entitled Highly Conductive Carrier Particles. The metal coatedcarrier particles of this invention generally have a resistance of lessthan 10 ohms with preferred materials having a resistance of less than 1ohm.

Conductive carrier as described above are excellent for use inmagnetic-brush development wherein it is desired to obtain a developmentelectrode effect thus producing solid area development. However, ifsolid area development is not desired, then it is necessary to suppressthis development electrode effect. The simplest way to accomlish thisresult is through the use of highly insulating carrier particles. Wheninsulating carrier particles are used, the development electrode effectis suppressed and fringing development occurs.

In accordance with this invention, high resistance iron carrierparticles can be provided by acid washing and rinsing, as referred toabove, followed by treatment to provide a uniform, adherent iron oxidesurface of high resistivity on the individual particles. However, inaccordance with this embodiment of the invention, subsequent washings toremove residual acid and Waste products are conducted without theaddition of any antioxidant material. After water washing, the iron isdrained to remove surplus moisture and then dried without subsequentsolvent treatment. The drying is conducted at elevated temperatures soas to induce formation of surface oxides. Good results are obtained withdrying temperatures of from about 40 to about C. During the dryingoperation, the damp powder is mixed gently in order to keep thetemperature and water content relatively uniform throughout the mass ofpowder. During drying, the color of the iron gradually change from grayto brown. After drying, the treated powder exhibits a very high level ofsurface resistivity. The optimum drying conditions (i.e., temperature,agitation, humidity, etc.) needed to produce the desired thin, uniformoxide coatings will be dependent upon the particular iron powder used asa starting material. Simple experimentation will readily show whichconditions are optimum for a particular powder chosen. In general, goodresults are obtained with air drying temperatures in the range of fromabout 40 to about 80 C. with a preferred temperature being in the rangeof from about 45 to about 60 C. The relative humidity of the drying aircan vary widely with good results being obtained at values of 40% RH.and above. Preferably, the drying air has a relative humidity of 60% andabove. Oxide-coating particles can be prepared in this manner havingelectrical resistance ranging up to greater than about 10 ohms.

When high resistance carrier particles of the above type are mixed withan appropriate toner material, the resultant developer composition isfound to greatly reduce the development electrode effect of a magneticbrush. In addition, such a developer composition induce fringingdevelopment with the accompanying increase in exposure latitude. Theiron oxide coating on these carriers can also be overcoated with a thinplastic film to further increase the resistance of the carrier andthereby inducing greater fringe development. In particular, theparticles can be overcoated with continuous, uniform layers offilm-forming electrically insulating resinous material. The polymers orresinous material useful in overcoating the carrier particles of thisinvention can be selected from a variety of subtances. Useful resinsmust be film-forming and, in general, they are electrically insulatingsuch that when coated in desired amount they will impart the requisitedegree of electrical resistance to the carrier particles. Useful resinsinclude those capable of being cured, hardened, or otherwiseinsolubilized such that when a subsequent resin layer is applied thereis no substantial permeation of a resin layer into an adjacent layer.Suitable resins include thermosetting resins which harden upon theapplication of heat as Well as light sensitive resins which harden uponexposure to actinic radiation. Also combinations of resins havingdiverse inherent solubility characteristics can be used. The differenttypes of useful resins can be used alone or in various combinations.

Useful light sensitive or light-hardening resins would include polymericderivatives of cinnamic acid such as polyvinyl cinnamate, cinnamoylatedpolystyrene, ethylene vinyl cinnamate copolymer, cellulose cinnamate, N-(cinnamoylphenyl)-methane derivatives of hydroxylated polymers (e.g.,partially hydrolyzed polyvinyl acetate cellulose acetate, etc.), epoxyresins esterified with cinnamoyl chloride and the like. Polyvinylcinnamate succinate and polyvinyl cinnamate phthalate would beparticularly easy to use in that they can be coated from an aqueoussolution. In addition, photocrosslinkable compositions comprising anunsensitive resin and a light sensitive crosslinking agent are useful.Such compositions could comprise an alcohol soluble nylon-type polyamidewith, for example, benzophenone as an initiator, etc.

Useful thermosetting or heat-hardening resins include a wide variety ofmaterials with formaldehyde condensation products being exemplary ofreadily available materials. Particularly useful materials areformaldehyde condensation products formed with urea, melamine or withvarious phenols such as xylenol, cresol, trimethylphenol, phenol,soligenin, etc. Other resins can also be used such as thermosettingepoxy resins, polyester-styrene resins and the like.

Other useful resins would include cellulose resins, such as cellulosenitrate, cellulose acetate butyrate, etc., and lower alkyl methacrylatepolymers having from 1 to 4 carbon atoms in the alkyl moiety, etc. Ingeneral, filmforming polyesters, polyolefins, polyamides,polycarbonates, etc., can all be used provided they are applied fromsuitable liquid vehicles which will not soften any previously appliedlayer. Methods for applying multiple layers of such resins are disclosedin co-pending Miller US. application, Ser. No. 799,968, filed Feb. 17,1969, co-filed herewith, and entitled High Resistance Carrier Particles.Of course, single layers of useful polymeric materials are also useful.Resin overcoated particles prepared in this manner generally have anelectrical resistance of greater than about ohms.

Electroscopic developer compositions can be prepared by mixing fromabout 90 to about 99% by weight of the present acid-washed magneticallyresponsive carrier particles with from about 10 to about 1% by weight ofa suitable electroscopic toner material. The toner used with the carrierparticles of this invention can be selected from a wide variety ofmaterials to give desired physical properties to the developed image andthe proper triboelectric relationship to match the carrier particlesused. Generally, any of the toner powders known in the art are suitablefor mixing with the carrier particles of this invention to form adeveloper composition. When the toner power selected is utilized withferromagnetic carrier particles in a magnetic-brush developmentarrangement the toner clings to the carrier by triboelectric attraction.The carrier particles acquire a charge of one polarity and the toneracquires a charge of the opposite polarity. Thus, if the carrier ismixed with a resin toner which is higher in the triboelectric series,the toner normally acquires a positive charge and the carrier a negativecharge.

Toner powders suitable for use in this invention are typically preparedby finely grinding a resinous material and mixing with a coloringmaterial such as a pigment or a dye. The mixture is then ball milled forseveral hours and heated so that the resin flows and encases thecolorant. The mass is cooled, broken into small chunks and finely groundagain. After this procedure the toner powder particles usually range insize from about 0.5 to about 25 microns, with an average size of about 2to about 15 microns.

The resin material used in preparing the toner can be selected from awide variety of materials, including natural resins, modified naturalresins and synthetic resins. Exemplary of useful natural resins arebalsam resins, colophony and shellac. Exemplary of suitable modifiednatural resins are colophony-modified phenol resins and other resinslisted below with a large proportion of colophony. Suitable syntheticresins are all synthetic resins known to be useful for toner purposes,for example, polymers, such as vinyl polymers includingpolyvinylchloride, polyvinylidene chloride, polyvinyl acetate, polyvinylacetals, polyvinyl ether and polyacrylic and polymethacrylic esters;polystyrene and substituted polystyrenes or polycondensates, e.g.,polyesters such as phthalate resin, terephthalic and isophthalicpolyesters, maleinate resin and colophony-mixed esters of higheralcohols; phenolformaldehyde resins, including colophony-modified phenolformaldehyde condensates, aldehyde resins, ketone resins, polyamides andpolyadducts, e.g., polyurethanes. Moreover, polyolefins, such as variouspolyethylenes, polypropylenes, polyisobutylenes and chlorinated rubberare suitable. Additional toner materials which are useful are disclosedin the following US. patents: 2,917,460; Re. 25,136; 2,788,288;2,638,416; 2,618,552 and 2,659,670.

The coloring material additives useful in suitable toners are preferablydyestuffs and colored pigments. These materials serve to color the tonerand thus render it more visible. In addition, they sometimes affect, inknown manner, the polarity of the toner. In principle, virtually all ofthe compounds mentioned in the Color Index, vol. I and II, Secondedition, 1956, can be used as colorants. Included among the vast numberof suitable colorants would be such materials as Nigrosin Spirit soluble(CI. 50415), I-Iansa Yellow G (CI. 11680), Chromogen Black BT00 (C.I.14645), Rhodamine B (C.I. 45170), Solvent Black 3 (CI. 26150), FuchsineN (CI. 42510) C.I. Basic Blue 9 (CI. 52015), etc.

The following examples are included for a further understanding of theinvention and all indications of mesh sizes have reference to the US.Standard Sieve Series:

EXAMPLE 1 A 2500 g. quantity of a commercial iron powder (Glidden Iron388, Glidden-Durkee Div. SCM Corp.) is poured into 1500 ml. of 2 Nsulfuric acid solution over a 10- second period. The iron powder usedhas a particle size such that it will pass through a mesh screen and beretained by a mesh screen. The resistance of the iron carrier asmeasured in the standards resistance test referred to above is 2400ohms. After addition of the iron to the sulfuric acid, stirring iscontinued for 60 seconds whereupon the reaction mixture is diluted to4000 ml. with cold tap water. The mixture is allowed to settle for 10seconds and then, the brown, supernatant liquid is decanted along with aconsiderable quantity of black greasy scum on the surface. Decantationrinsing is repeated 8 more times with 2000 ml. volumes of water. Theslurry is agitated after each addition of rinse water so as to brieflysuspend all iron powder. After suspension, the powder is allowed tosettle and the supernatant liquid is again decanted along with suspendeddirt and other unwanted materials. After the final water rinse, the ironis drained and rinsed 4 times with 600 ml. volumes of anhydrous methanolwith thorough mixing after each addition followed by a settling periodof 10 seconds prior to decantation. Following the final alcohol rinse,the drained iron is suction filtered on a Biichner funnel and theresulting damp powder is dried in a thin layer inch or less) on a glasssheet. The powder is mixed continuously during drying with heat beingprovided by infrared radiation from two 250 watt infrared bulbs at adistance of about 45 cm. from the powder. The two infrared bulbs providesufiicient heat to prevent the powder from being cooled excessively byevaporation of the residual methanol. The ambient air is at atemperature of about 21 C. and a relative humidity of slightly less thanabout 40%. Upon drying, the resistance of the powder was measured in thestandard test referred to above and found to be 16 ohms. The dry powderweighs 2468 grams which represents a total loss of about 1.3% in dirt,surface oxide, pyrites, etc., as well as extremely finely divided ironpowder decanted in washing. A magnet placed in all of the dirt, etc.,removed from the starting iron attracted only a minute amount ofmaterial indicating that only very little of the material lost isactually iron. Next, two magnetic brush developer compositions areprepared from the acid-washed iron and with an equal quantity ofuntreated starting material. The two carriers are each mixed with 3% byweight of a black, styrene co-polymer toner, having a particle size offrom 1-5 microns. The toner material charges positively on the carrierparticles. The two developer mixtures are tested in a magnetic-brushdevelopment apparatus. The apparatus for testing comprises a cylindricalaluminum tube arranged to rotate axially in a horizontal position abouta fixed permanent magnet. The permanent magnet has its poles orientedsuch that when the magnetic particles are present, a magnetic brush isformed on top of the cylinder. In the test procedure, this apparatus isrun for 15 minutes with the magnetic brush in contact with anelectrophotographic element comprised of a conductive support coatedwith a photoconductive layer of an organic photoconductor in a resinbinder. The developer formed using the untreated carrier particles showsconsiderable tendency to leave a scum or deposit on the photoconductorafter repeated use; whereas, the developer containing acid-treatedcarrier showed very little tendency to leave a scum. In'addition, whenthe two carrier particles are tested in the same apparatus using notoner, the acid-treated carrier particle produces no visible depositduring the test; while the untreated carrier deposits a black layerhaving a density of about 0.8. The two developer mixtures are then usedto develop an electrostatic latent image carried on a photoconductiveelement as described above. The developer containing the acid-treatedcarrier gives better overall image quality with more solid areadevelopment while showing less sensitivity to variations in theelectrical potential applied to the magnetic brush.

EXAMPLE 2 A quantity of 454 g. of high density electrolytic iron powder(Glidden 475, Glidden-Durkee Div. SCM Corp.) is poured rapidly withstirring into 250 ml. of a 5% hydrochloric acid solution in a 1 literbeaker. The iron powder has a particle size such that it will passthrough a 60 mesh screen and be retained by a 120 mesh screen. Inaddition, the resistance of this iron powder as measured in the standardresistance test described above is 640,000 ohms. The slurry of ironpowder and hydrochloric acid is mixed continuously for 2 minutes atwhich time the reaction of the acid on the iron powder subsides. Theacidtreated iron is then rinsed 4 times by decantation with 750 cc. of a0.5% solution of sodium nitrite in distilled water. After the last waterrinse, the iron is drained and rinsed twice with anhydrous ethanol.Excess alcohol is then removed by filtration using a Biichner funnel forminutes. The damp powder is rapidly dried by sprinkling it repeatedlythrough a current of dry air at a temperature of approximately 30 C. anda relative humidity of less than 30%. The powder is fully dried withinabout 4 minutes. The yield is 450 grams. After drying, the acid-treated10 powder is measured for resistance in the standard resistance test andfound to be 0.2 ohm. Next, the acid-treated carrier particles are placedin the mechanical magneticbrush apparatus as described in Example 1. Theapparatus is run using the treated carrier particles for a period of 15minutes. At the end of this period no visible scum is deposited on theelectrophotographic element used. An equivalent quantity of uncoatedcontrol carrier material when used in the test apparatus for 15 minutesproduces a deposit on the photoconductive element which measures 0.25 indensity using a standard densitometer. Next, the acid-treated carrierand the control carrier are mixed with 5% portions of a blue tonerpowder and again subjected to the standard scumming test. The developerprepared with the acid-washed carrier particles produced a slight amountof scum which is barely visible; whereas, the control developer producesa large amount of scum which is readily visible. In addition, thecontrol developer when in use is subject to a large degree of tonerthrowoif while the developer of the present invention exhibitsconsiderably less toner throw-off. The two developer compositions arethen used to develop an electrostatic latent image carried on anelectrophotographic element. The developer containing the acid-washedcarrier gives good image quality with excellent solid area developmentboth with and without bias on the magnetic brush. The control developergives some image development; however, the solid areas are developed ina very uneven manner. The

. hue of the two developed images shows the image formed from thecontrol developer to be less saturated than the image formed from thedeveloper containing the acidwashed carrier. The developer containingthe acid-treated carrier is then placed in an open 1 liter beaker andheld for 3 weeks under average room conditions (about 20 C. and 45%relative humidity). At the end of this period, the developer is measuredfor resistance and found to be substantially unchanged from theresistance value at the start of the test period. In addition, when thisdeveloper is tested again for image development, the results obtainedare substantially the same as those obtained when using a freshlyprepared developer composition in accordance with this example.

EXAMPLE 3 A 5 kilogram quantity of spherical iron particles, of a. sizethat will pass through a mesh screen and be retained by a 200 meshscreen, is poured into 3 liters of a 1:15 dilution of concentratednitric acid at 20 C. The mixture is slurried for 2 minutes and thendiluted to 10 liters with tap water at about 5 C. The diluted mixture isallowed to settle for about 10 seconds and the supernatant liquid andsuspended contaminants are decanted. The initially black iron powderappears silver-gray in color after this acid treatment. The acid-treatediron is then subjected to 5 decantation rinses using tap water at 20 C.whereupon thesurplus water is drained from the iron powder by suction ina Btichner funnel. The damp iron powder is spread out to a thickness ofabout 1 inch in a plastic tray and allowed to dry. Four 250 wattinfrared bulbs are positioned approximately 50 cm. from the tray to warmthe powder during drying. During the drying operation, the iron isturned over continuously while maintaining an average depth of about 2%cm. As the powder dries, it turns progressively more brown in color. Theweight of the dry product is 4986 grams. After drying, the resistance ofthe material is measured and found to be 2X10 ohms as compared to 2.8 10ohms for the starting material. A hand-held bar magnet is then used toattract the treated carrier material. The amount of carrier materialwhich is picked up by the bar magnet is measured and the magnet iscleaned and repeated again using the starting iron particles. It isfound that the bar magnet will attract 5% by Weight more of the treatedcarrier material than the untreated material. Next, two developercompositions are prepared using the treated carrier and the controlcarrier. Each developer contains 4% by weight of a black toner powderhaving an average particle size of 2 microns and comprising a nigrosinecolorant in a styrene polymeric binder. The two developer compositionsare then used in the mechanical brush apparatus described in Example 1.The acid-treated carrier containing developer produces images of goodquality with excellent fringe development being obtained. In addition, awide exposure latitude is obtained with the developer containingacid-treated carrier. The control developer gives only very limitedexposure latitude and only slight fringing development.

iron leaflets having a particle size such that they will pass through a60 mesh and be retained by an 80 mesh screen is placed into the bottomof a glass tube having an inside diameter of about 3 cm. and about 60cm. long. One liter of trichloroacetic acid solution is pumped upwardthrough the iron in the tube. The solution is circulated through theiron powder 4 times in 5 /2 minutes. Suspended dirt and othercontaminants are trapped in a wool-felt filter before recirculation ofthe solution. The acid is then replaced with cold tap water which isforced upward through the bed of iron powder at a rate sufiicient toremove unwanted small particles of contaminants without carrying awayany appreciable amount of the screened carrier material. The water flowrate is approximately 1 liter per 100 seconds. After minutes of washing,the wet material is transferred to a suction funnel to remove surplusmoisture. The top of the funnel is provided with a plastic cap toexclude air and nitrogen is pumped into the cap at a rate sufiicient toprevent collapse of the cap. After 30 minutes, the nearly dried powderis removed and finally dried rapidly by sprinkling it repeatedly througha current of dry air at about C. and a relative humidity of about 40%.The resistance of the final product as measured in the standard test asdscribed above is 37 ohms compared to 1200 ohms for the startingmaterial. Equal quantities of the starting material and the acidtreatedmaterial are used to prepare developer compositions, each containing 3%by weight of the black toner of Example 1. When used to developelectrostatic latent images, both developers give solid area developmentin a standard magnetic brush apparatus; however, the developer with theacid-treated highly conductive carrier produces substantially bettersolid area fill-in and is less sensitive to changes in the electricalbias of the magnetic brush than is the control developer.

EMMPLE 5 A 1600 g. quantity of commercial iron powder in the form offlattened grains (Glidden Iron 412, Glidden- Durkee Div. SCM Corp.) isacid-treated as follows. The

starting iron powder has a nominal particle size such that it will passthrough a 60 mesh screen and be retained by a 120 mesh screen; however,the powder has an extremely large portion (about 15%) of much morefinely divided material. The starting material is placed on a 120 meshplastic screen and sprayed with an acid solution comprising 200 cc. ofconcentrated sulfuric acid and 3 liters of tap water at about 15 C. Theiron is spread out evenly over an area of about 700 sq. cm. and sprayeduniformly with the acid at a rate of about ml. per second under about 10pounds pressure through a plastic spray nozzle. The force of the sprayjet helps to separate small contaminants and force them down through theplastic screen. Next, the powder is rinsed with a 0.2% solution ofhydrazine sulfate. After about 10 liters of the rinse solution has beensprayed over the iron, the material is drained, rinsed free of surpluswater by spraying with 2 liters of isopropanol and dried in a current ofdry air at a temperature of about 32 C. and a relative humidity of about35%,. The velocity of the air is maintained at a sufficient rate toproduce continuous movement of the powder during drying. In addition,further agitation is provided by brushing the iron during the dryingstep with a small brush. After the iron is completely dry, the air isturned off, but manual brushing action is continued for 3 minutes toassist in sifting out any particles having a size smaller than 120 mesh.The resistance of the treated material is measured and found to be 7ohms as compared to 900 ohms for the starting material. Next, thecarrier material is mixed with 3% by weight of a yellow, styrene basetoner which charges negatively with respect to the iron carrierparticles. The resulting developer composition is used to developpositively charged electrostatic latent images carried on anelectrophotographic element. Excellent solid area development isobtained and the resultant image is of a high quality and has abrilliant yellow hue. A control developer prepared with the same yellowtoner and the starting iron carrier, which has been screened to reducethe number of fine particles, gives images of poor quality havingsubstantially less solid fill-in and considerably less hue saturation.This loss of color brightness appears to be caused by loose dirt andoxide on the surface of the control carrier material, which dirt mixeswith toner and is transferred to the electrostatic image. The controldeveloper is also undesirable in that it is much more sensitive tochange in bias potential on the magnetic brush.

EXAMPLE 6 Two kilograms of iron treated as in Example 1 above are coatedwith a thin continuous layer of nickel by electroless plating for 30minutes at about C. in a bath of the following composition:

Nickel(ous)chloride N-Cl -6H O67.5 g. Sodium citrate Na C H O -2HO-123.0 g. Ammonium chloride NH C175.0 g.

28% ammonia solution150.0 ml.

Sodium hypophosphite NaH PO 'H O-16.5 g. Water to make 1500.0 ml.

The nickel-plated powder is removed from the bath and washed with sixchanges of cold water, rinsed three times with ethanol, suction filteredand dried at room temperature. The resistance of the dry powder ismeasured in the standard resistance test and found to be less than /2ohm. The nickel-coated carrier is then used to prepare a developercomposition as in Example 1. Electrostatic latent images developed withthis developer show excellent solid area fill-in.

EXAMPLE 7 A 700 g. quantity of a stainless steel iron powder having aparticle size such that it will pass through a 200 mesh screen and beretained by a 325 mesh screen is treated with acid in the same manner asin Example 1 using 500 ml. of a 1:19 aqueous solution of sulfuric acidfollowed by 12 decantation rinses with 1500 ml. quantities of water at38 C. After the last water rinse, the residual water is drained out byplacing the iron in a suspended cotton sack for 15 minutes. The iron isthen placed in a shallow glass tray and dried with a current of air atroom temperature while keeping the level of the iron at about 1%: cm.depth and turning over the iron every two or three minutes with a handspatula. During the course of drying, the iron acquires a gray color.The final product is measured in the standard resistance test and foundto have a resistance of 10 megohms as compared to 22,000 ohms for thestarting material. The result carrier is used to prepare a developercomposition and used to develop an electrostatic latent image as inExample 1. Good fringing development is obtained using this developer. Asimilar developer composition prepared using the control carrier and thetoner of Example 1 gives neither good solid area nor good fringingdevelopment.

13 EXAMPLE 8 A one kilogram quantity of iron powder as prepared inExample 3 is mixed with 300 ml. of a 7% solution of polyvinylcinnamatelight sensitive resin in methyl Cellosolve acetate. The powder is spreadout on a glass surface and dried at room temperature. The dried powderis then exposed to a 35 ampere arc lamp for 10 minutes whilecontinuously stirred to insure uniform exposure. This procedure isrepeated four times resulting in four distinct resin coatings on eachparticle. The particles are then measured in the standard resistancetest and'found to have a resistance value of greater than about 10 ohms.A developer composition is prepared with this carrier and used todevelop an electrostatic latent image as in Example 1. Excellent fringedeveloped images are obtained over a wide range of exposures.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

I claim:

1. A process for preparing a magnetically responsive developercomposition for use in developing electrostatic charge patternscomprising the steps of dispersing iron powder in a dilute aqueousmineral acid bath having a concentration of about 1 N to about 3 N,agitating the powder while in the bath, separating the powder from thebath, rinsing the powder with water, drying the wet powder in anoxygen-containing atmosphere at a temperature of from about 45 to about70 C. whereby a thin uniform layer of iron oxide is produced on thesurface of the individual particles of iron powder, overcoating eachparticle of iron powder with at least one thin, uniform layer of anelectrically insulating, filmforming polymer to form magneticallyresponsive carrier particles having average particle sizes from about1200 to about 40 microns and having an electrical resistance of greaterthan about 10 ohms and mixing from about 90 to about 99% by weight ofthe resultant carrier particles with from about 10 to about 1% by weightof an electroscopic toner material having a particle size smaller thanthat of the carrier particles.

2. A process for preparing a magnetically responsive developercomposition for use in developing electrostatic charge patternscomprising the steps of dispersing iron powder comprised of particleshaving an average diameter of from about to 600 microns, in a diluteaqueous mineral acid bath having a concentration of about 1 N to about 3N, agitating the powder while in the bath, separating the powder fromthe bath, rinsing the powder with water, drying the wet powder in anoxygen-containing atmosphere at a temperature of from about 45 to aboutC., whereby a thin, uniform layer of iron oxide is produced on thesurface of the individual particles of iron, overcoating each particleof iron with at least one thin, uniform layer of an electricallyisulating, film-forming polymer to form magnetically responsive carrierparticles having an electrical resistance of greater than about 10 ohms,said polymer being selected from the group consisting of cinnamateresins, cellulose esters, polyesters, polyolefins, polyamides,polycarbonates, formaldehyde condensation products and loweralkylmethacrylate resins and mixing from about to about 99% by weight ofthe resultant carrier particles with from about 10 to about 1% by weightof an electroscopic toner material having a particle size smaller thanthat of the carrier particles.

References Cited UNITED STATES PATENTS 2,890,968 6/1959 Giaima 25262.13,121,642 2/1964- Biskup 117--100 3,166,444 1/1965 Ehren 11749 3,278,43910/1966 Blanchette et a1. 25262.1

GEORGE F. LESMES, Primary Examiner J. P. BRAMMER, Assistant Examiner US.Cl. X.R. 1l717.5, 100

